‫Well come to the microphone interface explain lecture in this lecture I'm going to explain the output
‫from a microphone and how we can read the signals with an FPGA.
‫First off one is a microphone.
‫Everyone is probably familiar with what a microphone is has one used one or heard one.
‫And actually that's what we're using right now to talk to you guys and microphone.
‫But what does a microphone actually physically do.
‫We know we talk to a microphone and it makes it broadcasts our voice over speakers correct.
‫Well not necessarily the microphone is simply a sound transducer and all that means is a microphone
‫takes and sound waves such as your voice and turn them into voltages.
‫And so the output from a microphone is going to be a signal that changes really fast just depending
‫upon the pitch or the volume of your voice.
‫So how do we interface a microphone with an FPGA.
‫Well just like a temperature sensor we need to have an analog to digital conversion that takes place
‫because the output of a microphone is an analog signal.
‫And so you have to either have an external ADC chip for an internal ADC if your FPGA supports that and
‫when the microphone puts an output the louder your voice the higher the output will be.
‫And depending on the frequency of your voice will determine the frequency of that signal.
‫And by taking samples of the microphone at regular intervals we can gain an idea of what that actual
‫signal looks like from a digital perspective.
‫Some will throw a little digital signal processing theory at you.
‫Here we have the Nyquist theory which is also known as the sampling theorem and it states that you must
‫sample at a minimum of twice the rate of a signal you're trying to accurately sample.
‫So what does that mean for a microphone.
‫We're assuming that we're trying to capture the human voice and humans can hear typically in the range
‫of 20 hertz to 20 kilohertz.
‫So to accurately sample a microphone and to be able to recreate that signal we need a sample at a rate
‫of twice 20 kilohertz which would be 40 kilohertz.
‫And if you notice when you use any type of software with like audio recording typically will have a
‫rate of 44 kilohertz and that's because you want to be at least minimum of twice the sampling rate.
‫So they'll sometimes say OK 40 is the minimum we'll go up to 44 just to be sure.
‫And there's a few people who can say they can hear higher than 20 kilohertz.
‫Now if we under sample the audio let's say we don't reach the full 40 kilohertz we sample at 20 kilohertz
‫or 30 kilohertz per se.
‫If we do this we won't be able to actually accurately recreate that signal.
‫We need to know what the highest frequency of the signal we are trying to capture is so that we can
‫accurately recreate that signal and get an understanding of what's going on.
‫If we're if we're under sampling and we think that something else is that is occurring and we're not
‫getting the full picture.
‫This is called aliasing and that's an digital signal processing.
‫They'll use the term aliasing now over sampled audio.
‫You cannot over sample audio where it hurts you per se.
‫If we say we need to have a signal sampled at 44 kilohertz we could sample it at 400 kilo Hertz 500
‫kilohertz 600 kilohertz.
‫It's not going to hurt you or you're just going to throw more samples on that signal and you actually
‫get a better idea of what's actually occurring.
‫And some people when you're working with audio signals they'll claim that when you're sampling at a
‫much higher rate it makes the audio sound much more pure or clear.
‫However to me at 44 kilohertz I can't tell the difference of that sample are higher.
‫Now what will happen if you over sample audio.
‫It's going to cost you in terms of processing time.
‫If you had only a much faster DSP microprocessor or a PDA in his course or with PJ's to run faster means
‫we're going to utilize more power or Also more resources so we can do more things in parallel.
‫So you want to sample high enough to recreate the signal.
‫What you don't want to oversample so much that it costs you in terms of resources and processing time.
‫Now that you know how to interface your FPGA with the microphone.
‫Let's get started on the assignment.